The present invention relates to paper-manufacturing machines.
In particular, the present invention relates to paper manufacturing machines designed to manufacture tissue paper.
The machine to which the present invention relates includes a twin-wire forming section preceded by a single-wire forming section which serves to decrease the amount of dewatering which is required at the twin-wire forming section. Thus, a pair of endless fabrics, each of which may take the form either of a wire or felt, are lapped around a rotary forming roll to form a twin-wire former therewith, and one of these endless fabrics provides in advance of the forming roll a single-wire former with which, preferably, a dewatering structure such as a forming board cooperates. A headbox, which is preferably a hydraulic headbox, cooperates with that one of the endless fabrics where the single-wire web formation takes place to supply pulp stock thereto.
In the manufacture of tissue paper it is well known to form the web on a relatively short wire section resembling a standard planar wire to which the headbox supplies the pulp stock at the region of a breast roll which is often formed with openings at its surface or which is provided with an internal vacuum, the web forming formation being carried by the wire past conventional dewatering members such as table rolls, deflectors, foil laths, suction boxes, and a suction roll, each of which removes water from the web. At the end of this wire section the partially dried web is transferred onto a pick-up felt, to be supported thereby while proceeding further towards the press and drying sections where additional drying takes place. In certain known constructions the above planar wire section has been entirely omitted. In this case the formation of the web is initially carried out entirely on a wire-covered suction breast roll from which the web is directly transferred onto the pick-up felt.
Conventional constructions of the above type have a drawback in that, for example, the upper speed limit of the paper machine will be about 1500 meters per minute. This relatively low maximum speed is necessitated because a relatively high draining pressure is essential, and this high draining pressure in turn causes an excessively great adherence of the web to the wire with resulting difficulties being encountered in connection with the detachment of the web from the wire.
A further drawback of convention structures of the above type resides in the fact that the use of a separate pick-up means results in losses of energy as well as of fibers, and increases the first cost of the machine.
Furthermore, conventional machines of the above type have certain limitations with respect to the use of inferior quality raw materials such as ground wood, deciduous wood, and waste paper pulp, owing to poor initial wet strength and also to poor ultimate strength. This drawback is primarily due to the well-known pick-up difficulties (occurrence of holes, web transfer, etc.) as well as to difficulties encountered in the reeling of the paper and its further conversion.
Several types of twin-wire formers intended for the manufacture of tissue paper are well known. Most of these known twin-wire formers are so-called full throat formers, but these structures generally have the drawback that as a result of the throat forming process there is a poor formation of the web, and the web becomes excessively adherent to the wire, this latter undesirable result being caused by the extremely high dewatering rate at the initial web-forming stage. Moreover, with such structure difficulties are encountered at the web-transfer steps.
With respect to the prior art reference may be made to U.S. Pat. Nos. 3,378,435 and 3,537,954, the first of these patents disclosing a construction generally known as as a "Crescent" former.
Also, reference may be made to U.S. Pat. No. 3,997,390 which discloses single-wire formation as an initial procedure with the possibility of controlling various process variables already used by practicing paper makers on Fourdrineer paper machines. In this connection the most essential process variables are the pulp stock discharge velocity with respect to the wire, the angle of incidence of the stock jet on the wire, and the rate at which water is drained from the suspension which is deposited on the wire.